This invention relates to the partial oxidation (partial combustion) of hydrogen sulphide and in particular to a method of and apparatus for forming sulphur vapour by partial oxidation of hydrogen sulphide.
Hydrogen sulphide containing gas streams (sometimes referred to as xe2x80x9cacid gas streamsxe2x80x9d) are typically formed in oil refineries and natural gas processing units. Such streams cannot be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. Thus, the hydrogen sulphide is effectively partially oxidised. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the sulphur dioxide takes place. Resulting sulphur vapor is extracted downstream of each stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, e.g. by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere.
Most Claus plants are equipped with right cylindrical furnaces having a length to internal diameter ratio in the range of from 2:1 to 4:1. The furnaces may be cross-fired or tangentially-fired by a burner or burners mounted at the side. Cross or tangentially fired burners achieve good mixing of the reacting chemical species. If desired, mixing can be enhanced by providing the furnace with baffles or checkerwork walls.
Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of given size. This ceiling can be raised by using commercially pure oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.
If commercially pure oxygen or oxygen-enriched air having a mole fraction of oxygen above 0.65 is used to support the combustion of the hydrogen sulphide there is a relatively high risk of damage to the refractory lining of the furnace being created by the resulting increase in flame temperature depending on the composition of the Claus feed gas. There are a number of proposals in the art to solve this problem. Some proposals involve introduction of flame moderators such as water into the furnace; others involve recycling to the furnace gas from a downstream part of the plant so as to moderate the temperature in the furnace; and yet others employ a plurality of furnaces so as to limit the amount of combustion that is performed in each individual furnace, thereby obviating the need for an external flame moderator or to recycle gas from a downstream part of the plant. All these proposals, however, add to the complexity of the plant.
Axially or longitudinally fired burners mounted on the back wall may be used instead of cross or tangentially fired burners in Claus furnaces. Such axially or longitudinally fired burners can be designed to provide average residence times comparable with those of crossxe2x80x94or tangentiallyxe2x80x94fired burners at a specified throughput, and may be preferred at higher levels of oxygen-enrichment.
The use of such an axially or longitudinally fired burner is disclosed in European patent application 0 315 225 A, in which there is a central pipe for oxygen, at least one second pipe for hydrogen sulphide containing feed gas which coaxially surrounds the central pipe, and an external coaxial pipe for air. The burner is used when the hydrogen sulphide feed gas contains at least 5% by volume of carbon dioxide or hydrocarbons. The oxygen velocity at the outlet of the burner is in the range of from 50 to 250 metres per second (typically 150 metres per second) and the corresponding feed gas velocity is in the range of 10 to 30 metres per second. Temperatures in the range of 2000 to 3000xc2x0 C. are generated in the core of the burner flame, and a gas mixture having a temperature in the range of 1350 to 1650xc2x0 C. leaves the furnace. This gas mixture contains at least 2% by volume of carbon monoxide and at least 8% by volume of hydrogen.
During normal operation of, for example, an oil refinery the rate at which hydrogen sulphide containing gas streams are produced for treatment by the Claus process is not constant and can vary quite widely. It is therefore desirable that the furnace be capable of effective operation over a wide range of different rates of inflow of the hydrogen sulphide containing gas.
WO-A-96/26157 also discloses the use of an axially or longitudinally fired burner in the Claus process. Generally parallel flows of a first gas containing hydrogen sulphide and a second gas enriched in oxygen are supplied to the tip (i.e. mouth) of the burner. The ratio of the velocity of the first gas to the velocity of the second gas is selected so as to be in the range of 0.8:1 to 1.2:1.
Neither EP-A-0 315 225 A nor WO-A-96/26157 discusses the problem of how to handle a wide range of different rates of inflow of the hydrogen sulphide containing gas. In fact, neither discloses a method which is capable of effective operation if the rate of supply of the feed gas containing hydrogen sulphide varies considerably.
The method and apparatus according to the invention have it as aim to address this problem and to provide a solution superior to any possible when the disclosure of EP-A-0315 255 or WO-A-96/26157 is followed.
It is the primary object of the present invention to provide a method of forming sulphur by the partial oxidation of hydrogen sulphide.
According to the present invention there is provided a method of forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising operating a burner so as to establish a flame having at least three stages in a furnace in or into which the burner fires, supplying to the flame from a first region of the mouth of the burner at least one flow of a first combustible gas comprising hydrogen sulphide, causing at least one second flow of a first oxidizing gas to issue from the mouth of the burner and mix in the flame with the first combustible gas, supplying to the flame from a second region of the mouth of the burner surrounding and spaced from said first region at least one third flow of a second combustible gas comprising hydrogen sulphide, causing at least one fourth flow of a second oxidizing gas to issue from a region or regions of the mouth of the burner surrounded by said second region and mix in the flame with the second combustible gas, causing at least one fifth, outermost, flow of a third oxidizing gas to mix in the flame with the second combustible gas, and withdrawing from the furnace a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, hydrogen and residual hydrogen sulphide.
The invention also provides apparatus for forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising a furnace, a port in the furnace, a burner having its mouth located in the port and operable to establish a flame having at least three stages in the furnace, and an outlet from the furnace for a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, and residual hydrogen sulphide to exit the furnace; wherein the mouth of the burner has a first outlet or group of outlets for supplying to the flame at least one first flow of a first combustible gas comprising hydrogen sulphide, a second outlet or group of outlets for causing at least one second flow of a first oxidizing gas to issue from the burner and mix in the flame with the first combustible gas, a third outlet or group of outlets, surrounding and spaced apart from the first outlet or group of outlets, for supplying to the flame at least one third flow of a second combustible gas comprising hydrogen sulphide, and a fourth outlet or group of outlets surrounded by said third outlet or group of outlets for causing at least one fourth flow of a second oxidizing gas to issue from the burner and mix in the flame with the second combustible gas; wherein a passage or passages are defined between the burner and the port, or extend through or terminate in the mouth of the burner, and are able to cause an outermost fifth flow of a third oxidizing gas to mix in the flame with the second combustible gas.
Burning the hydrogen sulphide in three stages, namely an innermost stage, an outermost stage, and an intermediate stage, makes it possible to handle effectively a wider range of different rates of inflow of the hydrogen sulphide containing gas than if one or two such stages are employed. Other advantages accrue from such staging of the combustion. In particular, a relatively low temperature can be maintained in the outermost stage even though a temperature in excess of 2000xc2x0 C. may be created in innermost stage, and therefore risk of damage to any refractory lining of the furnace can be kept to acceptable levels. A high temperature, that is a temperature well in excess of 2000xc2x0 C., is particularly advantageous because it facilitates destruction of any ammonia in the first combustible gas and creation of conditions which increase the proportion of the resulting sulphur vapor that is formed directly by thermal cracking of hydrogen sulphide rather than by the indirect route involving oxidation of some hydrogen sulphide or sulphur to sulphur dioxide and then reaction of the thus formed sulphur dioxide with residual hydrogen sulphide. Destruction of ammonia is desirable because this gas tends to affect adversely downstream processing of the effluent from the furnace in catalytic reactors in which hydrogen sulphide and sulphur dioxide react together to form further sulphur vapor, the ammonia acting to block the catalyst by formation of ammonium salts. Moreover, the ammonia can be destroyed in the flame without resort to a combustion zone and a reaction zone separate from one another with some of the amine gas by-passed directly to the reaction zone. By avoiding the need to by-pass amine gas around the combustion zone even when using air for combustion, the ability of the burner to handle effectively a wide range of different flow rates of feed gases is enhanced due to a more effective use of its turndown range. In addition, forming sulphur vapor by thermal cracking reduces the rate at which oxygen-enriched air needs to be supplied to the burner to achieve a given recovery of sulphur vapor from the furnace, provided the furnace gases are cooled effectively downstream of the furnace.
Preferably the flame extends generally longitudinally within the furnace. The furnace is typically disposed with its longitudinal axis horizontal, and therefore the burner is typically also disposed with its longitudinal axis horizontal. Such arrangements can help to keep down the risk of damage to any refractory lining employed in the furnace.
The first and second oxidizing gases preferably have a mole fraction of at least 0.22 and may be oxygen-enriched air or pure oxygen. The third oxidizing gas is preferably atmospheric air neither enriched in nor depleted of oxygen, although enrichment up to 25% by volume of oxygen, or higher depending on the composition of the feed, is generally acceptable.
Preferably the mass flow rate of the first combustible gas and the mass flow rate of the second combustible gas to the burner are controlled independently of one another. Such an arrangement facilitates operation of the burner to handle variations in the total rate at which it is desired to feed combustible gas to the burner. The apparatus according to the invention therefore preferably additionally includes a first flow control valve in a first pipeline for supplying the first combustible gas to the burner, and a second flow control valve in a second pipeline for supplying the second combustible gas to the burner, the first and second control valves being operable independently of one another.
In a typical refinery there is more than one source of combustible gas comprising hydrogen sulphide. The sources typically have different compositions. Preferably the first combustible gas is of a different composition from the second combustible gas. By this means it is possible to optimise combustion of the combustible gas. Typically, both the first and second combustible gas streams contain at least 40% by volume of combustibles and at least 20% by volume of hydrogen sulphide.
If there are two separate sources of combustible gas comprising hydrogen sulphide, one containing ammonia, the other not, then all the ammonia containing gas is preferably employed in forming the first combustible gas. As a result it becomes possible to direct all the ammonia to a relatively inner region of the flame where a relatively high flame temperature can be maintained in order to destroy all the ammonia. For example, if one source of gas containing hydrogen sulphide is so-called xe2x80x9csour water stripper gasxe2x80x9d, which typically contains about 20 to 35% by volume of hydrogen sulphide and 30 to 45% by volume of ammonia, and another source of gas containing hydrogen sulphide is so-called xe2x80x9camine gasxe2x80x9d which typically contains over 80% by volume of hydrogen sulphide, the first combustible gas may comprise a mixture of some of the amine gas but all of the sour water stripper gas, and the second combustible gas may comprise the remaining amine gas. Preferably, the composition of the mixture is varied with the total rate of flow of combustible gas comprising hydrogen sulphide to the flame, with the proportion of amine gas in the first combustible gas being increased if the said total rate of flow is reduced below a chosen value.
Preferably the mass flow rate of the first oxidizing gas and the mass flow rate of the second oxidizing gas to the burner are controlled independently of one another. Such an arrangement facilitates operation of the burner to handle variations in the total rate at which it is desired to feed combustible gas to the burner and to cater for changes in the individual mass flow rates of the first and second combustible gas streams. The apparatus according to the invention preferably additionally includes a third flow control valve in a third pipeline for supplying the first oxidizing gas to the burner, and a fourth flow control valve in a fourth pipeline for supplying the second oxidizing gas to the burner. The third and fourth control valves are operable independently of one another.
The first and second oxidizing gases may be taken from the same or different sources of oxidizing gas. If different sources are employed, the first oxidizing gas may have a different composition from the second oxidizing gas. Employing first and second oxidizing gases of different composition adds to the flexibility of the method and apparatus according to the invention in effectively handling variable rates of supply of combustible gas.
The mole fraction of oxygen in both the first and second oxidizing gas is typically in the range of 0.3 to 1.0 depending on the proportion of combustibles in the first and second combustible gases. Care should be taken to avoid creating an excessive temperature at any location in any refractory employed to line the furnace. Modern commercially available refractories can typically withstand temperatures up to 1650xc2x0 C. The oxygen-enriched air or pure oxygen of which either or both of the first and second oxidizing gases may be composed may be taken directly from an air separation plant. Depending on the purity of the oxygen product of the air separation plant, either or both of the first and second oxidizing gases may have a mole fraction of oxygen greater than 0.99. In general, however, particularly when handling sour water stripper gas, or amine gas, or mixtures of the two, it is preferred to form either or both of the first oxidizing gas and the second oxidizing gas by mixing an oxygen product of the air separation plant with atmospheric air, that is air which is neither enriched in nor depleted of oxygen. Forming either or both of the first and second oxidizing gases in this way makes it possible to vary the mole fraction of oxygen during operation of the method and apparatus according to the invention. Again, this ability to vary the mole fraction of oxygen adds to the flexibility of the method and apparatus according to the invention in effectively handling varying rates of supply of combustible gas.
Mixing of the first combustible gas with the first oxidizing gas is preferably facilitated by directing at least some of the first oxidizing gas along a path or paths which meet a path or paths followed by the first combustible gas. Accordingly, the second outlet or at least some of the second group of outlets each have an axis which extends at an angle to the axis of the first outlet or the axes of at least some of the second group of outlets. The angle is preferably in the range of 10 to 30xc2x0. Preferably, the flow of the first combustible gas is axial and the flow of the first oxidizing gas is at an angle to the axis of the burner.
Alternatively, mixing of the first combustible gas with the first oxidizing gas can be facilitated by directing at least some of the first oxidizing gas at a first linear velocity along a path or paths extending generally contiguous and generally parallel to a path or paths followed by the first combustible gas at a second linear velocity, and one of the first and second linear velocities is from 25 to 150% (and preferably from 25 to 100%) greater than the other thereof. Mixing is facilitated because the differential velocity between the first oxidizing gas and the first combustible gas creates forces of shear therebetween. Preferably, it is the first linear velocity which is selected to be the greater of the two said linear velocities. This arrangement facilitates design of the furnace to ensure that all the ammonia is destroyed in it.
A further alternative for facilitating mixing of the first combustible gas with the first oxidizing gas is to impart a swirling motion to one or both of the first oxidizing gas and the first combustible gas. Devices that are able to impart swirl to the gas are well known.
The natural curvature of the flame tends to facilitate mixing of the said fourth flow of second oxidizing gas flow with the said third flow of the second combustible gas. Nevertheless, it is preferred to arrange the supply of the said third and fourth flows so as to facilitate mixing. For example, at least some of the second oxidizing gas may flow at an angle to the second combustible gas such that the flow paths intersect, or at least bring the two gases closer together. Preferably, the second combustible gas leaves the mouth of the burner essentially axially, and at least some of the second oxidizing gas leaves the mouth of the burner at an angle to the axis. The angle is typically in the range of 10 to 30xc2x0 to the axis of the burner. In another arrangement the third and fourth flows leave the burner alongside one another and at different velocities such that shear therebetween aids mixing.
If the total flow of the combustible gases becomes less than the maximum for which the burner is designed, then a reduction may be made in the flow of the first and second oxidizing gases and in the mole fraction of oxygen in both oxidizing gases. Further, in order to maintain a high temperature at the core of the flame when the burner is turned down, the proportional reduction in the rate at which the second combustible gas is supplied to the burner may be greater than that in the flow rate of the first combustible gas. In addition, if the first combustible gas is a mixture of sour water stripper gas and amine gas, the proportion of amine gas in the mixture may be changed. For this purpose, the first pipeline may communicate with a source of sour water stripper gas and the second pipeline with a source of amine gas, there being a conduit placing the second pipeline in communication with the first pipeline, and a further flow control valve in the further conduit for controlling flow of amine gas into the first pipeline.
In one preferred burner according to the invention there is a first tube defining a first passageway for the first combustible gas which terminates in the first outlet and within which extend a plurality of second tubes defining second passageways for the first oxidizing gas which each terminate in a respective second outlet, and a third tube surrounding and generally coaxial with the first tube, and defining therewith an annular third passageway for the second combustible gas which terminates in the third outlet and a plurality of fourth tubes for the second oxidizing gas which each extend within the third passageway and which define fourth passageways each terminating in a respective fourth outlet. Such an arrangement permits adjustment of the flow of the first oxidizing gas independently of the flow of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the flow of the second combustible gas, and vice versa.
Alternative preferred arrangements are possible. In one such arrangement, the first and second tubes are provided and disposed as described above. In addition, there is a third tube concentric with and surrounding the first tube to define therewith an annular third passageway for flow of the second oxidizing gas, the third passageway terminating in a nozzle in which the fourth outlets are formed, and a fourth tube concentric with and surrounding the third tube to define therewith an annular fourth passageway for the second combustible gas terminating in the said third outlet. Such an arrangement also permits adjustment of the flow of the first oxidizing gas independently of the flow of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the flow of the second combustible gas, and vice versa.
In yet another preferred arrangement, there are four concentric radially spaced apart tubes defining a central tubular passageway, and innermost intermediate, and outermost annular passageways. The central tubular passageway ends in the first outlet and the outermost annular passageway in the fourth outlet. The other two passageways both end in respective nozzles, the nozzles defining the second and third groups of outlets. This arrangement is another which permits adjustment of the flow of the first oxidizing gas independently of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the second combustible gas and vice versa.
In less preferred burners there may be three concentric tubes defining a central tubular passageway and inner and outer passageways. The central tubular passageway ends in the first outlet and the outer annular passageway in the third outlet. The inner annular passageway terminates in a nozzle in which both the second and fourth group of outlets are defined. Such an arrangement does not permit the flow rate and composition of the first oxidizing gas to be adjusted independently of the flow rate and composition of the second oxidizing gas.
The resultant gas mixture is preferably cooled in a waste heat boiler, and the cooled effluent gas stream is preferably passed to a condenser in which sulphur vapor is condensed therefrom. The resultant gas stream is preferably subjected downstream of the sulphur condenser to at least one stage of catalytic reaction between hydrogen sulphide and sulphur dioxide so as to enable further sulphur to be extracted.